1. Field of the Invention
The invention relates to cyclodextrin glycosyltransferases which primarily produce .gamma.-cyclodextrin and which, with starch as substrate, form primarily .gamma.-cyclodextrin and as secondary products almost exclusively cyclic oligosaccharide.
2. The Prior Art
The enzyme cyclodextrin glycosyltransferase (abbreviation: CGTase) E.C.2.4.1.19 catalyzes the formation of cyclodextrins from starch. Depending on the number of glucose units of which the cyclodextrin (CD) is composed, a distinction is made between .alpha.-CD (6 glucose units), .beta.-CD (7 glucose units) and .gamma.-CD (8 glucose units).
To date, two types of CGTase have been disclosed:
a) CGTases which primarily form .alpha.-cyclodextrin, also called .alpha.-CGTase, such as, for example, the CGTase from Bacillus macerans (U.K. Patent No. 2,169,902), from Klebsiella pneumoniae (EPA 220,714) and from Bacillus stearothermophilus (U.K. Pat. No. 2,169,902).
b) CGTases which primarily form .beta.-cyclodextrin, or .beta.-CGTase, such as, for example, the CGTase from Bacillus circulans (U.S. Pat. No. 4,477,568), from Bacillus megaterium (U.S. Pat. No. 3,812,011), from Bacillus ohbensis (Japan Patent No. 74,124,285), from Micrococcus sp. (EPA No. 017,242) and from alkalophilic Bacillus sp. (J. Gen. Microbiol. 1988, 134, 97-105; Appl. Microbiol. Biotechnol. 1987, 26, 149-153).
Concerning a .gamma.-CGTase, to date there have been two indications in the literature:
a) The cyclodextrin glycosyltransferase from Bacillus sp. produces on addition of EtOH predominantly a mixture of .beta.- and .gamma.-cyclodextrin (10.4 to 18.7%) (Japan Patent No. 63,133,998). The enzyme has not been characterized in terms of its kinetic properties, so that it cannot be assigned to a specific type.
b) Two articles (Agric. Biol. Chem., 1986, 50, (8), 2161=2162 and Denpun Kagaku 1986, 33, 137) describe a .gamma.-CGTase from Bacillus subtilis No. 313. This CGTase is distinguished by the formation of .gamma.-cyclodextrin and linear oligosaccharide. Since CGTases generate only cyclic products from starch, this "CGTase" is a transitional form between an .alpha.-amylase (generates linear oligosaccharide from starch) and a CGTase. This ".gamma.-CGTase" is unsuitable for preparing .gamma.-cyclodextrin because only low yields can be achieved (see EPA 327,099, page 2, lines 40-43).